1. Field of the Invention
The present invention relates to a steering apparatus for automotive vehicles, and more particularly to an improvement of the steering apparatus for enhancing a straight running characteristic of the vehicle.
2. Description of the Prior Art
Disclosed In Japanese Utility Model Laid-open Publication Hei No. 1-144170 is a steering apparatus of the rack-and-pinion type for an automotive vehicle which includes a tubular housing mounted on the vehicle body to support a rack member displaceable in a lateral direction, a pair of tie rods each connected to the opposite ends of the rack member to steer a set of road wheels in accordance with lateral displacement of the rack member, and a pair of dust boots each coupled at their one ends with the opposite ends of the tubular housing and at their other ends with the tie rods to encompass each joint portion of the tie rods with the rack member.
In such a conventional steering apparatus as described above, the steering characteristic is symmetrical in left and right directions to maintain the steering wheel in a neutral position even if the driver causes tie vehicle to travel on a flat road without holding the steering wheel. In general, the surface of the flat road is, however, raised at a central portion of the road for drainage. Accordingly, if the driver causes the vehicle to travel on the flat road without holding the steering wheel placed in the neutral position, the steering wheel will be displaced leftward or rightward from the neutral position in accordance with lateral inclination of the road surface. As a result, straight travel of the vehicle may not be maintained. In the case that the road wheels of the vehicle are equipped with radial tires for traveling on a left or right lane of the travel road, a ply-steer residual cornering force caused by the tires themselves acts to restrain the lateral displacement of the vehicle. If the ply-steer residual cornering force is excessive, the vehicle tends to travel in an opposite direction against the lateral inclination of the road surface.